Inner Focus Lens System and Image Pickup Apparatus Using the Same

ABSTRACT

An inner focus lens system comprises, in order from an object side to an image side, a first lens unit, an aperture stop, a second lens unit having a positive refractive power, a third lens unit having a negative refractive power, and a fourth lens unit having a positive refractive power, wherein the first lens unit has at least one negative lens and at least one positive lens, the second lens unit has at least one negative lens and at least one positive lens, the third lens unit has at least one negative lens, the fourth lens unit has at least one positive lens, at the time of focusing on an object at a short distance from an object at infinity, the third lens unit moves to an image side so as to lengthen a distance to the second lens unit and to shorten a distance to the fourth lens unit, and the following conditional expression (1) is satisfied: 
       −6&lt; f   3   /f &lt;−1   (1).

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 14/576,908, filed on Dec. 19, 2017, which is a divisionalapplication of U.S. patent application Ser. No. 13/707,758, filed onDec. 7, 2012, now U.S. Pat. No. 8,947,793, issued on Feb. 3, 2015, whichis based upon and claims the benefit of priority from the prior JapanesePatent Application No. 2011-279434 filed on Dec. 21, 2011, the entirecontents of each of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an inner focus lens system and an imagepickup apparatus using the same.

Description of the Related Art

Conventionally, a so-called Gauss type wide-angle lens has been known asa wide-angle lens used for a photographic camera and a still videocamera. A Gauss type lens system is a lens system in which a refractivepower arrangement is substantially symmetric across an aperture stop.The Gauss type lens system includes lens systems described in JapanesePatent Application Laid-Open No. 2003-241084 and Japanese PatentApplication Laid-Open No. 2009-258157. The lens systems described inJapanese Patent Application Laid-Open No. 2003-241084 and JapanesePatent Application Laid-Open No. 2009-258157 are wide-angle lenses atabout F2.8.

Further, as a similar Gauss type lens system, there is a lens systemdescribed in Japanese Patent Application Laid-Open No. 2009-237542. Thelens system described in Japanese Patent Application Laid-Open No.2009-237542 is a large-diameter (about F1.8) standard lens. Further, asa lens system which has realized a further larger diameter (about F1.4),there is a lens system described in Japanese Patent ApplicationLaid-Open No. 2010-66432.

In the Gauss type lens systems as described in the above publications ofunexamined applications, a lens unit having a positive refractive poweris located on an image side than an aperture stop, and the entire lensunit having this positive refractive power is brought up (moved towardan object side). Thus, the Gauss lens systems as described in thepublications of unexamined applications are lens systems employing aso-called rear focus method.

SUMMARY OF THE INVENTION

An inner focus lens system of the present invention comprises, in orderfrom an object side to an image side,

a first lens unit,

an aperture stop,

a second lens unit having a positive refractive power,

a third lens unit having a negative refractive power, and

a fourth lens unit having a positive refractive power,

wherein a total number of lens units in the inner focus lens system isfour, which is the first lens unit, the second lens unit, the third lensunit, and the fourth lens unit,

wherein the first lens unit has at least one negative lens and at leastone positive lens,

the second lens unit has at least one negative lens and at least onepositive lens,

the third lens unit has at least one negative lens,

the fourth lens unit has at least one positive lens,

at the time of focusing on an object at a short distance from an objectat infinity,

the third lens unit moves to the image side so as to lengthen a distanceto the second lens unit and to shorten a distance to the fourth lensunit, and

the following conditional expression (1) is satisfied:

−6<f ₃ /f<−1   (1)

where f3 is a focal length of the third lens unit, and

f is a focal length of the entire inner focus lens system at the time offocusing on the object at infinity.

Further, an image pickup apparatus of the present invention comprises:

an inner focus lens system, and

an image pickup device located on an image side of the inner focus lenssystem and converting an image formed by the inner focus lens systeminto an electrical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B are cross-sectional views of lenses in an innerfocus lens systems of the present invention at the time of focusing onan object at infinity; FIG. 1A is a cross-sectional view of lenses of afirst example, and FIG. 1B is a cross-sectional view of lenses of asecond example.

FIG. 2A, FIG. 2B, FIG. 2C, FIG. 2D, FIG. 2E, FIG. 2F, FIG. 2G, FIG. 2H,FIG. 2I, FIG. 2J, FIG. 2K, and FIG. 2L are aberration diagrams of theinner focus lens system of the first example, and aberration diagrams ofthree different focusing states.

FIG. 3A, FIG. 3B, FIG. 3C, FIG. 3D, FIG. 3E, FIG. 3F, FIG. 3G, FIG. 3H,FIG. 3I, FIG. 3J, FIG. 3K, and FIG. 3L are aberration diagrams of theinner focus lens system of the second example, and aberration diagramsof three different focusing states.

FIG. 4 is a view to explain a half angle of view.

FIG. 5 is a cross-sectional view of a lens-exchangeable camera using aninner focus lens system according to the present invention as anexchangeable lens.

FIG. 6 is a front perspective view illustrating an appearance of adigital camera according to the present invention.

FIG. 7 is a rear perspective view of the digital camera of FIG. 6.

FIG. 8 is a configuration block diagram of an internal circuit of a mainpart of the digital camera of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

The following describes an inner focus lens system of the presentembodiment.

The inner focus lens system of the present embodiment comprises, inorder from an object side to an image side, a first lens unit, anaperture stop, a second lens unit having a positive refractive power, athird lens unit having a negative refractive power, and a fourth lensunit having a positive refractive power, wherein the first lens unit hasat least one negative lens and at least one positive lens, the secondlens unit has at least one negative lens and at least one positive lens,the third lens unit has at least one negative lens, the fourth lens unithas at least one positive lens, at the time of focusing on an object ata short distance from an object at infinity, the third lens unit movesto an image side so as to lengthen a distance to the second lens unitand to shorten a distance to the fourth lens unit, and the followingconditional expression (1) is satisfied:

−6<f ₃ /f<−1   (1)

where f₃ is a focal length of the third lens, and

f is a focal length of the entire inner focus lens system at the time offocusing on the object at infinity.

In the inner focus lens system of the present embodiment, the lens unitsare located such that the first lens unit, the aperture stop, the secondlens unit having a positive refractive power, and the third lens unithaving a negative refractive power are located in this order from theobject side. At the time of focusing on the object at a short distancefrom the object at infinity, the third lens unit is moved to the imageside.

In the inner focus lens system of the present embodiment, the first lensunit and the second lens unit each include at least one negative lensand at least one positive lens, and the aperture stop is located betweenthe first lens unit and the second lens unit. With such a configuration,main aberration correction is performed by lens units located on theobject side than the third lens unit, namely, by the first lens unit andthe second lens unit. This makes it possible to suppress a burden due tothe aberration correction in the third lens unit. Here, as describedabove, since focusing is performed by the third lens unit, such aconfiguration is advantageous to reduction of the number of lenses inthe lens unit to perform focusing, and the like.

Further, when a diameter of the lens system becomes large, aberrationfluctuation due to the movement of the third lens unit at the time offocusing is easy to become large. Particularly, fluctuation of sphericalaberration and curvature of field is easy to become large. In order tocorrect (reduce) this aberration fluctuation, it is preferable to makethe refractive power of the third lens unit small to some extent.

However, when the refractive power of the third lens unit is made small,a focus driving amount (a moving amount of the third lens unit) at thetime of focusing on the object at a short distance from the object atinfinity is increased. Besides, as the moving amount is larger, a heightof an off-axis light beam at a position of the third lens unit becomeshigher, thereby resulting in that a lens diameter of the third lens unitbecomes large. As such, if the refractive power of the third lens unitis made small, it is difficult to downsize the third lens unit.

In view of this, in the inner focus lens system of the presentembodiment, the fourth lens unit is located on the image side than thethird lens unit. At the time of focusing on an object at a shortdistance from an object at infinity, the third lens unit is moved to theimage side so as to lengthen a distance to the second lens unit and toshorten a distance to the fourth lens unit.

The fourth lens unit is provided, so that the fourth lens unit can bearthe correction of aberration fluctuation at the time of focusing on theobject at a short distance. Further, since the fourth lens unit isprovided, it is unnecessary for the refractive power of the third lensunit to be made small, so that the moving amount of the third lens unitat the time of focusing can be made small. As a result, the lens systemcan be downsized and a diameter of the lens system becomes large, at thesame time, and further, it is easy to suppress upsizing of the lenssystem when a wide field angle is ensured.

In addition, a refractive power of the fourth lens unit is a positiverefractive power, so that a back focus can be shortened and the lenssystem can be downsized. Further, it is preferable that the refractivepower of the fourth lens unit be a positive refractive power, in view ofexcellent correction of a Petzval sum and chromatic aberration.

As such, it is preferable that a total number of lens units in the innerfocus lens system be four, which is the first lens unit, the second lensunit, the third lens unit, and the fourth lens unit.

Further, it is preferable that the inner focus lens system of theembodiment satisfy the following conditional expression (1):

−6<f ₃ /f<−1   (1)

Where f₃ is a focal length of the third lens, and

f is a focal length of the entire inner focus lens system at the time offocusing on the object at infinity.

When a focal length of the third lens unit is defined by the conditionalexpression (1) appropriately, it is possible to suppress a size of theentire lens system and aberration fluctuation at the time of focusing onthe object at a short distance, to be small.

If the lower limit of the conditional expression (1) is not reached, thefocal length of the third lens unit becomes too large. In this case, themoving amount of the third lens unit at the time of focusing becomeslarge, which is disadvantageous to downsizing of the lens system. Inaddition, the Petzval sum becomes large, which makes it difficult tocorrect curvature of field and chromatic aberration. Thus, it isdisadvantageous to the correction of these aberrations to be not reachedthe lower limit of the conditional expression (1).

If the upper limit of the conditional expression (1) is exceeded, thefocal length of the third lens unit becomes too small. In this case, itis difficult to correct aberration in the entire lens system, andspherical aberration and coma at the time of focusing on the object at ashort distance. Thus, it is disadvantageous to the correction of theseaberrations to exceed the upper limit of the conditional expression (1).

Further, it is preferable that the inner focus lens system of theembodiment satisfy the following conditional expression (2):

0.5<f ₂ /f<3   (2)

where f₂ is a focal length of the second lens, and

f is a focal length of the entire inner focus lens system at the time offocusing on the object at infinity.

When the focal length of the second lens unit is defined by theconditional expression (2) appropriately, the entire length of the lenssystem can be shortened and aberration can be corrected well.

To prevent below the lower limit of the conditional expression (2), arefractive power of the second lens unit can be suppressed from beingtoo large. As a result, the occurrence of spherical aberration andchromatic aberration in the entire lens system can be reduced. Thus, itis advantageous for the reduction in the occurrence of these aberrationsto prevent below the lower limit of the conditional expression (2).

To prevent above the upper limit of the conditional expression (2), anappropriate refractive power can be ensured in the second lens unit. Asa result, the lens system can be downsized. Thus, it is advantageous forthe downsizing of the lens system to prevent above the upper limit ofthe conditional expression (2).

Further, if a diameter of the lens system becomes large and an angle ofview of the lens system is widened, spherical aberration and sagittalcoma flare occur. Particularly, in such a configuration that the firstlens unit has only one negative lens and one positive lens in order froman object, an amount of spherical aberration occurring when the diameterof the lens system becomes large and the angle of view is widened and anamount of sagittal coma flare occurring when the angle of view iswidened become large. Because of this, in the configuration with onenegative lens and one positive lens, it is difficult to correct theseaberrations in the first lens unit.

In view of this, in the inner focus lens system of the presentembodiment, the first lens unit has a plurality of negative lenses andat least one positive lens, and a lens on the most object side in thefirst lens unit is any one of the plurality of negative lenses.

Since the first lens unit has a plurality of negative lenses, that is,at least two negative lenses, spherical aberration and sagittal comaflare occurring in the positive lens can be corrected well. Further,since the negative lenses are located on the most object side, even ifthe angle of view is widened, an appropriate back focus can be ensured.As a result, the large diameter and the wide angle of view can beachieved in the lens system.

Further, in the inner focus lens system of the present embodiment, it ispreferable that the first lens unit have at least four lenses.

Since the first lens unit has at least four lenses, various aberrationsin the first lens unit and the entire lens system can be corrected well.

Further, in the inner focus lens system of the present embodiment, it ispreferable that the first lens unit have a cemented lens in which anegative lens and a positive lens are cemented together, on the imageside than the negative lens on the most object side.

Since the first lens unit includes a cemented lens of a negative lensand a positive lens separately from the negative lens on the most objectside, chromatic aberration can be corrected well. Further, when thefirst lens unit has four lenses, various aberrations in the first lensunit and the entire lens system can be corrected well.

Further, in the inner focus lens system of the present embodiment, it ispreferable that the second lens unit include a negative lens, a positivelens, and a positive lens in order from the object side.

Since the second lens unit includes a negative lens and positive lensesin order from the object side, the second lens unit accordingly includesa retrofocus-type lens configuration. As a result, a principal pointposition of the second lens unit can be positioned rather on the imageside. Further, a sufficient back focus is ensured so that the height ofan off-axis light beam in the third lens unit can be lowered.

Further, since the positive lenses are located on the image side, thesecond lens unit is configured to include two positive lenses. Thisallows the two positive lenses to bear the refractive power, therebyresulting in that aberration can be corrected well in the second lensunit, and in addition, decentration sensitivity of each of the negativelens and the two positive lenses can be reduced. Further, since therefractive power of the second lens unit is easy to be strengthened bythe two positive lenses, it is advantageous for the downsizing of thelens system to include the two positive lenses.

Further, in the inner focus lens system of the present embodiment, it ispreferable that the third lens unit consist of one negative lens.

As mentioned earlier, the third lens unit is a lens unit to be used forfocusing. Therefore, if this lens unit is constituted by one lens, thelens unit can be reduced in weight. As a result, vibration and noise atthe time of focusing can be reduced. Besides, focusing drive can bespeeded up. Thus, it is advantageous for the reduction in vibration andnoise and speeding-up of the focusing drive to constitute the third lensunit by one negative lens.

Further, in the inner focus lens system of the present embodiment, it ispreferable that the fourth lens unit consist of one positive lens, or atmost two lenses including one positive lens and one negative lens.

It is advantageous for the reduction in the entire length of the lenssystem to constitute the fourth lens unit by at most two lenses.

Further, in the inner focus lens system of the present embodiment, it ispreferable that a refractive power of the first lens unit is a negativerefractive power.

It is advantageous for securing of an appropriate back focus that therefractive power of the first lens unit is a negative refractive power.

Further, in the inner focus lens system of the present embodiment, it ispreferable that an F number of the inner focus lens system be 2 or lessand that a half angle of view of a light beam incident at a maximumimage height position through a center of the aperture stop exceed 30°.

It is advantageous for securing of appropriate brightness and securingof a wide angle of view to set the F number to be 2 or less and to setthe half angle of view to exceed 30°. Further, this configuration allowsacquisition of an excellent image even by photographing in a dark placeor photographing in a room.

Here, the following describes the half angle of view with reference toFIG. 4. FIG. 4 illustrates a state in which an optical system, anaperture stop, and an image pickup surface are located on an opticalaxis. Light beams incident on the optical system pass through theaperture stop, and are then emanated from the optical system to reachthe image pickup surface. Here, the optical system in FIG. 4 is theinner focus lens system of the present embodiment. Further, ω denotes ahalf angle of view.

In FIG. 4, L shown in a full line denotes a light beam reaching a pointX in an effective image pickup area, among light beams passing through acenter of the aperture stop. This point X is a position farthest fromthe optical axis in the effective image pickup area. Here, since theeffective image pickup area is an area where an object image is formed,the point X is at a maximum image height position. Thus, the light beamL is a light beam incident at the maximum image height position in theeffective image pickup area through the center of the aperture stop. Asillustrated in FIG. 4, the half angle of view ω is represented by anangle formed by the light beam L and the optical axis.

Further, in the inner focus lens system of the present embodiment, it ispreferable that at the time of focusing on the object at a shortdistance from the object at infinity, positions of the first lens unit,the aperture stop, the second lens unit, and the fourth lens unit befixed.

It is advantageous for prevention of contaminant to the lens system andnoise reduction at the time of focusing drive that the positions of thefirst lens unit, the second lens unit, and the fourth lens unit arefixed (the first lens unit, the second lens unit, and the fourth lensunit stand still) at the time of focusing.

Further, in the inner focus lens system of the present embodiment, it ispreferable that an exit plane of the first lens unit and an incidenceplane of the second lens unit be concave toward an aperture-stop side.

It is advantageous for correction of the Petzval sum that the exit planeof the first lens unit and the incidence plane of the second lens unitare concave toward the aperture-stop side.

Further, it is preferable that the inner focus lens system of thepresent embodiment satisfy the following conditional expressions (3) and(4):

−0.3<f ₂ /f ₁<0.3   (3)

0.02<D _(s) /f ₂<0.7   (4)

where f₁ is a focal length of the first lens,

f₂ is a focal length of the second lens, and

D_(s) is a distance on the optical axis from the exit plane of the firstlens unit to the aperture stop.

The conditional expression (3) is a condition to define a range ofoptimum focal lengths of the first lens unit and the second lens unit.

To prevent below the lower limit of the conditional expression (3), therefractive power of the first lens unit does not become a large negativerefractive power. This makes it possible to easily suppress theoccurrence of spherical aberration and coma in the first lens unit.

To prevent above the upper limit of the conditional expression (3), therefractive power of the first lens unit does not become a large positiverefractive power. This is advantageous for securing of an appropriateback focus.

The conditional expression (4) is an advantageous condition to thereduction in the entire length of the lens system.

It is advantageous for securing of a space where a mechanical mechanismof the aperture stop is to be located to ensure a distance from thefirst lens unit to the aperture stop appropriately so as to preventbelow the lower limit of the conditional expression (4).

To prevent above the upper limit of the conditional expression (4) leadsto the reduction in the entire length of the lens system.

Further, an image pickup apparatus of the present embodiment includesthe inner focus lens system, and an image pickup device located on animage side of the inner focus lens system and converting an image formedby the inner focus lens system into an electrical signal.

If the aforementioned inner focus lens system is included, an imagepickup apparatus which takes advantage of this lens system can beprovided.

Further, in order that functions (working-effects) are ensured more, itis preferable that each of the above conditional expressions have thefollowing upper limit and lower limit.

In regard to the conditional expression (1):

-   The lower limit is preferably −4, further preferably −3.-   The upper limit is preferably −1.7, further preferably −2.5.

In regard to the conditional expression (2):

-   The lower limit is preferably 0.9, further preferably 1.2.-   The upper limit is preferably 2, further preferably 1.5.

In regard to the conditional expression (3):

-   The lower limit is preferably −0.2, further preferably −0.1.-   The upper limit is preferably 0.1, further preferably 0.

In regard to the conditional expression (4):

-   The lower limit is preferably 0.07, further preferably 0.1.-   The upper limit is preferably 0.5, further preferably 0.3.

Note that the above inner focus lens system may satisfy a plurality ofconfigurations at the same time. This is preferable in order to obtainan excellent inner focus lens system and an excellent image pickupapparatus. Further, preferable configurations can be optionallycombined. Furthermore, in regard to each conditional expression, only anupper limit or a lower limit in a numerical range of a more limitedconditional expression may be limited.

The following describes examples of the inner focus lens system and theimage pickup apparatus according to the present invention in detailbased on the drawings. It should be understood that the presentinvention is not limited by the examples.

The examples shown below deal with a bright inner focus lens systemhaving an angle of view of 60° or more and an F number of 2 or less.Further, this inner focus lens system is preferably used for a takinglens of a camera such as a digital still camera.

The following describes Examples 1 and 2 of the inner focus lens systemof the present invention. FIG. 1A and FIG. 1B are cross-sectional viewsof lenses at the time of focusing on an object at infinity according toExamples 1 and 2, respectively. In the figures, a first lens unit isrepresented by G1, an aperture stop is represented by S, a second lensunit is represented by G2, a third lens unit is represented by G3, afourth lens unit is represented by G4, a parallel plate is representedby C, and an image surface is represented by I. Note that in FIG. 1A andFIG. 1B, filters (a dust removal filter, an infrared cut filter, a lowpass filter) and a cover glass protecting an image pickup surface areillustrated as one optically equivalent parallel plate C.

The inner focus lens system of Example 1 is constituted by, in orderfrom an object side, a first lens unit G1 having a negative refractivepower, an aperture stop S, a second lens unit G2 having a positiverefractive power, a third lens unit G3 having a negative refractivepower, and a fourth lens unit G4 having a positive refractive power, asillustrated in FIG. 1A.

At the time of focusing on an object at a short distance from an objectat infinity, the first lens unit G1 stands still, the aperture stop Sstands still, the second lens unit G2 stands still, the third lens unitG3 moves to the image side, and the fourth lens unit G4 stands still.

The first lens unit G1 is constituted by, in order from the object side,a negative meniscus lens L1 facing its convex surface toward the objectside, a negative meniscus lens L2 facing its convex surface toward theobject side, a positive meniscus lens L3 facing its convex surfacetoward the object side, and a negative meniscus lens L4 facing itsconvex surface toward the object side. Here, the positive meniscus lensL3 and the negative meniscus lens L4 are cemented together. The secondlens unit G2 is constituted by a biconcave negative lens L5, a biconvexpositive lens L6, and a biconvex positive lens L7. Here, the biconcavenegative lens L5 and the biconvex positive lens L6 are cementedtogether. The third lens unit G3 is constituted by a negative meniscuslens L8 facing its convex surface toward the object side. The fourthlens unit G4 is constituted by a biconvex positive lens L9. An imageside surface (a light-beam exit plane) of the negative meniscus lens L4and an object side surface (a light-beam incidence plane) of thebiconcave negative lens L5 are both concave toward an aperture-stopside.

Aspheric surfaces are used for six surfaces in total, i.e., bothsurfaces of the negative meniscus lens L2, both surfaces of the biconvexpositive lens L7, and both surfaces of the biconvex positive lens L9.

The inner focus lens system of Example 2 is constituted by, in orderfrom an object side, a first lens unit G1 having a negative refractivepower, an aperture stop S, a second lens unit G2 having a positiverefractive power, a third lens unit G3 having a negative refractivepower, and a fourth lens unit G4 having a positive refractive power, asillustrated in FIG. 1B.

At the time of focusing on an object at a short distance from an objectat infinity, the first lens unit G1 stands still, the aperture stop Sstands still, the second lens unit G2 stands still, the third lens unitG3 moves to the image side, and the fourth lens unit G4 stands still.

The first lens unit G1 is constituted by, in order from the object side,a negative meniscus lens L1 facing its convex surface toward the objectside, a negative meniscus lens L2 facing its convex surface toward theobject side, a positive meniscus lens L3 facing its convex surfacetoward the object side, and a negative meniscus lens L4 facing itsconvex surface toward the object side and facing it concave surfacetoward an aperture stop S side. Here, the positive meniscus lens L3 andthe negative meniscus lens L4 are cemented together. The second lensunit G2 is constituted by a biconcave negative lens L5, a biconvexpositive lens L6, and a biconvex positive lens L7. Here, the biconcavenegative lens L5 and the biconvex positive lens L6 are cementedtogether. The third lens unit G3 is constituted by a negative meniscuslens L8 facing its convex surface toward the object side. The fourthlens unit G4 is constituted by a biconvex positive lens L9 and abiconcave negative lens L10. An image side surface (a light-beam exitplane) of the negative meniscus lens L4 and an object side surface (alight-beam incidence plane) of the biconcave negative lens L5 are bothconcave toward an aperture-stop side.

Aspheric surfaces are used for six surfaces in total, i.e., bothsurfaces of the negative meniscus lens L2, both surfaces of the biconvexpositive lens L7, and both surfaces of the biconvex positive lens L9.

Numerical data of each example described above is shown below. Apartfrom symbols described above, r denotes radius of curvature of each lenssurface, d denotes a distance between respective lens surfaces, nddenotes a refractive index of each lens for a d-line, and vd denotes anAbbe constant for each lens. Further, a focal length denotes a focallength of the entire system, FNO. denotes an F number, ω denotes a halfangle of view, and each of f1, f2 . . . is a focal length of each lensunit. Note that the entire length is a length which is obtained byadding a back focus to a distance from a lens forefront surface up to alens backmost surface. Further, fb (back focus) is a unit which isexpressed upon air conversion of a distance from the lens backmostsurface to a paraxial image surface.

Further, in regard to a focusing state, “Infinity” denotes a state whenit focused on an object at infinity, “Magnification is −1/10” denotes astate when it focused on an object at the time when the magnification is−1/10, and “Object-Image distance is 200 mm” denotes a state when itfocused on an object at the time when a distance from the object to animage is 200 mm. Note that a state where it focused on an object at ashort distance is, for example, a state of the “Object-Image distance is200 mm”.

When x is let to be an optical axis with a direction of traveling oflight as a positive (direction) , and y is let to be in a directionorthogonal to the optical axis, a shape of the aspheric surface isdescribed by the following expression.

x=(y ² /R)/[1+{1−(K+1) (y/R)²}^(1/2) ]+A4 y ⁴ +A6 y ⁶ +A8 y ⁸ +A10 y ¹⁰+A12 y ¹²

where, R denotes a paraxial radius of curvature, K denotes a conicalcoefficient, A4, A6, A8, A10, and A12 denote aspherical surfacecoefficients of a fourth order, a sixth order, an eight order, a tenthorder, and a twelfth order respectively. Moreover, in the asphericalsurface coefficients, ‘e-n’ (where, n is an integral number) indicates‘10^(−n),’.

EXAMPLE 1

Unit mm Surface data Surface no. r d nd νd Object plane ∞ ∞  1 19.7451.00 1.60300 65.44  2 11.316 2.85  3* 58.287 1.20 1.68893 31.08  4*26.720 4.67  5 16.110 2.78 1.92286 20.88  6 53.780 1.00 1.58144 40.75  723.507 4.18  8(stop) ∞ 3.78  9 −10.768 1.00 1.92286 20.88 10 56.607 4.121.83481 42.71 11 −16.919 0.10 12* 35.628 4.44 1.80610 40.92 13* −20.176Variable 14 39.086 1.00 1.68893 31.07 15 17.000 Variable 16* 33.338 3.801.80610 40.92 17* −186.188 12.76  18 ∞ 4.08 1.51633 64.14 19 ∞ 0.75Image plane ∞ (Image pickup surface) Aspherical surface data 3rd surfacek = 26.612 A4 = 3.06498e−04, A6 = −3.00399e−06, A8 = 1.42771e−08, A10 =6.05212e−11, A12 = −1.18433e−12 4th surface k = 1.305 A4 = 3.21226e−04,A6 = −2.66717e−06, A8 = −2.96071e−09, A10 = 3.39774e−10, A12 =−3.41764e−12 12th surface k = 0.000 A4 = −3.80048e−05, A6 = 4.95630e−07,A8 = −5.02094e−09, A10 = 2.18585e−11 13th surface k = 0.000 A4 =1.71864e−05, A6 = 4.31519e−07, A8 = −4.84879e−09, A10 = 2.20282e−11 16thsurface k = 0.000 A4 = 2.91468e−05, A6 = 1.40438e−07, A8 = 2.92331e−09,A10 = −6.23715e−11, A12 = 2.90363e−13 17th surface k = 0.000 A4 =3.25935e−05, A6 = −1.84853e−07, A8 = 8.71138e−09, A10 = −1.15676e−10,A12 = 4.72611e−13 Various data Focal length 16.02 Fno. 1.72 Angle offield 2ω 72.98 fb (in air) 16.20 Lens total length (in air) 59.18 Imageheight 10.82 In focus state Infinity d13 2.47 d15 4.59 Magnification is− 1/10 d13 3.30 d15 3.76 Object-Image distance is 200 mm d13 5.21 d151.85 Unit focal length f1 = −385.32 f2 = 19.42 f3 = −44.49 f4 = 35.35

EXAMPLE 2

Unit mm Surface data Surface no. r d nd νd Object plane ∞ ∞  1 21.6441.00 1.60300 65.44  2 12.026 2.12  3* 30.305 1.20 1.68893 31.08  4*17.732 4.99  5 17.328 2.69 1.92286 20.88  6 59.102 1.00 1.58144 40.75  729.113 5.04  8(stop) ∞ 3.51  9 −11.676 1.00 1.92286 20.88 10 85.427 3.911.83481 42.71 11 −16.784 0.10 12* 36.189 3.92 1.80610 40.92 13* −22.804Variable 14 44.009 1.00 1.68893 31.07 15 17.291 Variable 16* 28.209 4.951.80610 40.92 17* −40.225 0.10 18 −268.143 1.00 1.59270 35.31 19 36.68010.62  20 ∞ 4.08 1.51633 64.14 21 ∞ 1.74 Image plane ∞ (Image pickupsurface) Aspherical surface data 3rd surface k = 7.554 A4 = 2.07444e−04,A6 = −1.67966e−06, A8 = −6.42096e−09, A10 = 2.19295e−10, A12 =−1.74479e−12 4th surface k = −1.325 A4 = 2.82036e−04, A6 = −1.24246e−06,A8 = −1.91341e−08, A10 = 4.35944e−10, A12 = −3.35543e−12 12th surface k= 0.000 A4 = −3.54666e−05, A6 = 5.70299e−07, A8 = −6.52032e−09, A10 =2.52235e−11 13th surface k = 0.000 A4 = 1.19746e−05, A6 = 4.42517e−07,A8 = −5.27387e−09, A10 = 1.86903e−11 16th surface k = 0.000 A4 =2.31959e−05, A6 = 8.72773e−08, A8 = 4.98712e−10, A10 = −2.25790e−11, A12= 8.32884e−14 17th surface k = 0.000 A4 = 4.02466e−05, A6 =−1.35944e−07, A8 = 3.42392e−09, A10 = −4.82780e−11, A12 = 1.65637e−13Various data Focal length 16.00 Fno. 1.72 Angle of field 2ω 73.16 fb (inair) 15.06 Lens total length (in air) 59.18 Image height 10.82 In focusstate Infinity d13 2.00 d15 4.59 Magnification is − 1/10 d13 2.83 d153.76 Object-Image distance is 200 mm d13 4.74 d15 1.85 Unit focal lengthf1 = −306.62 f2 = 20.17 f3 = −41.98 f4 = 32.62

Aberration diagrams of Examples 1 and 2 described above are shown inFIG. 2A to FIG. 2L and FIG. 3A to FIG. 3L, respectively. Further, ineach diagram, “FIY” denotes a maximum image height.

In these aberration diagrams, respective FIG. 2A to FIG. 2D andrespective FIG. 3A to FIG. 3D show spherical aberration (SA),astigmatism (AS), distortion (DT), and chromatic aberration ofmagnification (CC) at the time of focusing on an object at infinity.

Further, respective FIG. 2E to FIG. 2H and respective FIG. 3E to FIG. 3Hshow spherical aberration (SA), astigmatism (AS), distortion (DT), andchromatic aberration of magnification (CC) when a magnification is−1/10.

Further, respective FIG. 21 to FIG. 2L and respective FIG. 3I to FIG. 3Lshow spherical aberration (SA), astigmatism (AS), distortion (DT), andchromatic aberration of magnification (CC) when an object-image distanceis 200 mm.

Next, the values of conditional expressions (1) to (4) in eachembodiment are shown below.

Conditional expressions Example 1 Example 2 (1) f₃/f −2.78 −2.62 (2)f₂/f 1.21 1.26 (3) f₂/f₁ −0.05 −0.07 (4) D_(s)/f₂ 0.22 0.25

FIG. 5 is a cross-sectional view of a single-lens mirrorless camera asan image pickup apparatus. As shown in FIG. 5, 1 denotes a single-lensmirrorless camera , 2 denotes an photographic lens system disposedinside a lens barrel, and 3 denotes a lens mount of the lens barrel. Thelens mount 3 enable to the photographic lens system 2 to be detachablefrom the single-lens mirrorless camera 1. A screw type mount or abayonet type mount is used as the lens mount 3. In this example, thebayonet type mount is used. Further, 4 denotes an image pickup elementsurface and 5 denotes a back monitor. Also, a small-size CCD (chargecoupled device) or a CMOS (complementary metal oxide semiconductor) isused as an image pickup element.

The inner focus lens system according to the present invention,described in the first example or the second example, is used as thephotographic lens system 2 of the single-lens mirrorless camera 1.

FIG. 6 and FIG. 7 show conceptual diagrams of a structure of the imagepickup apparatus according to the present invention. FIG. 6 is a frontperspective view showing an appearance of a digital camera 40 as animage pickup apparatus, and FIG. 7 is a rear perspective view showing anappearance of the digital camera 40. The inner focus lens systemaccording to the present invention is used for a photographic opticalsystem 41 of the digital camera 40.

The digital camera 40 according to the embodiment includes thephotographic optical system 41 positioned on a capturing optical path42, a shutter button 45, and a liquid-crystal display monitor 47. Whenthe shutter button 45 disposed on an upper portion of the digital camera40 is pressed, in conjunction with the pressing of the shutter button45, an image is captured through the photographic optical system 41 suchas the inner focus lens system according to the first example. An objectimage which has been formed by the photographic optical system 41 isformed on an image pickup element (photoelectric conversion surface)provided near an image forming surface. The object image which has beenreceived by the image pickup element is displayed as an electronic imageon the liquid-crystal display monitor 47 provided on a rear surface ofthe digital camera by a processing unit. Moreover, it is possible torecord the electronic image which has been captured in a recording unit.

FIG. 8 is a block diagram showing an internal circuit of main componentsof the digital camera 40. In the following description, the processingunit mentioned above includes components such as CDS/ADC section 24, atemporary storage memory section 17, and an image processing section 18.A storage unit includes a storage medium section 19.

As shown in FIG. 8, the digital camera 40 includes an operating section12, a control section 13 which is connected to the operating section 12,an imaging drive circuit 16 which is connected to a control-signaloutput port of the control section 13 via buses 14 and 15, the temporarystorage memory section 17, the image processing section 18, a storagemedium section 19, a display section 20, and a set-information storagememory section 21.

The temporary storage memory section 17, the image processing section18, the storage medium section 19, the display section 20, and theset-information storage memory section 21 are capable of inputting andoutputting data mutually via a bus 22. Moreover, a CCD 49 and theCDS/ADC section 24 are connected to the imaging drive circuit 16.

The operating section 12 includes various input buttons and switches,and imparts event information input from outside (user of camera) viathe input buttons and switches to the control section 13. The controlsection 13 is a central arithmetic processing unit such as a CPU with abuilt-in program memory which is not shown in the diagram, and controlsthe overall digital camera 40 according to a computer program which hasbeen stored in the computer program memory.

The CCD 49 is an image pickup element which is driven and controlled bythe imaging drive circuit 16, and which converts an amount of light foreach pixel of the object image which has been formed through thephotographic optical system 41 to an electric signal, and outputs to theCDS/ADC section 24.

The CDS/ADC section 24 is a circuit which amplifies the electric signalinput from the CCD 49, and also carries out analog-to-digitalconversion, and outputs image raw-data only for the amplification anddigital conversion carried out (Bayer data, hereinafter called as ‘RAWdata’) to the temporary storage memory section 17.

The temporary storage memory section 17 is a buffer such as a SDRAM, andis a memory unit which temporarily stores the RAW data output put fromthe CDS/ADC section 24. The image processing section 18 is a circuitwhich reads the RAW data which has been stored in the temporary storagememory section 17 or the RAW data which has been stored in the storagemedium section 19, and carries out electrically, various imageprocessing including a distortion correction based on image-qualityparameters which have been specified by the control section 13.

The recording medium section 19 in which, a recording medium in the formof a stick or a card with a flash memory is detachably mounted, recordsand maintains the RAW data which is transferred from the temporarystorage memory section 17 and image data which has been subjected toimage processing in the image processing section 18.

The display section 20 includes the liquid-crystal display monitor 47and displays operation menu, image data, and RAW data captured. Theset-information storage memory section 21 is provided with a ROM sectionin which various image-quality parameters are stored in advance, and aRAM section which stores the image-quality parameters which have beenread from the ROM section by an input and output operation of theoperating section 12.

The digital camera 40 which is structured in such manner, by adoptingthe inner focus lens system according to the present invention as thephotographic optical system 41, it possible to let to be an image pickupapparatus which is advantageous for wide angle of view, small-sizing andobtaining an image in high resolution without deteriorating imagequality.

As described above, the inner focus lens system and the image pickupapparatus according to the present invention are useful to obtain animage in high resolution without deteriorating image quality in a largearea.

What is claimed is:
 1. An inner focus lens system comprising, in orderfrom an object side to an image side, a first lens unit, a second lensunit having a positive refractive power, a third lens unit having anegative refractive power, and a fourth lens unit having a positiverefractive power, wherein a total number of lens units in the innerfocus lens system is four, which is the first lens unit, the second lensunit, the third lens unit, and the fourth lens unit, the inner focuslens system has an aperture stop, the first lens unit has a negativelens disposed closest to the image side in the first lens unit and apositive lens disposed the object side of the negative lens, the secondlens unit comprises, in order from the object side, a cemented lensincluding a biconcave negative lens and a biconvex positive lens, and abiconvex positive lens, and wherein a total number of lenses in thesecond lens unit is three, the third lens unit consists of one negativelens having an image side surface concave to the image side, the fourthlens unit comprises, in order from the object side, a biconvex positivelens having an aspherical surface and a negative lens, and wherein atotal number of lenses in the fourth lens unit is two, at a time offocusing on an object at a short distance from an object at infinity,the third lens unit moves to the image side so as to lengthen a distanceto the second lens unit and to shorten a distance to the fourth lensunit, at the time of focusing on the object at a short distance from theobject at infinity, positions of the first lens unit, the second lensunit, the fourth lens unit and the aperture stop are fixed, an F numberof the inner focus lens system is 2 or less, and an exit surface of thefirst lens unit and an incidence surface of the second lens unit areconcave.
 2. The inner focus lens system according to claim 1, whereinthe first lens unit has a plurality of negative lenses, and a lens on amost object side in the first lens unit is any one of the plurality ofnegative lenses.
 3. The inner focus lens system according to claim 2,wherein the first lens unit has at least four lenses.
 4. The inner focuslens system according to claim 2, wherein the first lens unit has acemented lens in which a negative lens and a positive lens are cementedtogether, on the image side than the negative lens on the most objectside.
 5. The inner focus lens system according to claim 1, wherein arefractive power of the first lens unit is a negative refractive power.6. The inner focus lens system according to claim 1, wherein a halfangle of view of a light beam incident at a maximum image heightposition through a center of the aperture stop exceeds 30°.
 7. The innerfocus lens system according to claim 1, wherein the followingconditional expression (1) is satisfied:−6<f ₃ /f<−1   (1) where f3 is a focal length of the third lens unit,and f is a focal length of the entire inner focus lens system at thetime of focusing on the object at infinity.
 8. The inner focus lenssystem according to claim 1, wherein the following conditionalexpression (2) is satisfied:0.5<f2/f<3   (2) where f2 is a focal length of the second lens unit, andf is a focal length of the entire inner focus lens system at the time offocusing on the object at infinity.
 9. The inner focus lens systemaccording to claim 1, wherein the aperture stop is disposed between thefirst lens group and the second lens group, the following conditionalexpressions (3) and (4) are satisfied:−0.3<f2/f1<0.3   (3)0.02<Ds/f2<0.7   (4) where f1 is a focal length of the first lens unit,f2 is a focal length of the second lens unit, and Ds is a distance on anoptical axis from an exit surface of the first lens unit to the aperturestop.
 10. An image pickup apparatus comprising: the inner focus lenssystem according to claim 1, and an image pickup device located on animage side of the inner focus lens system and converting an image formedby the inner focus lens system into an electrical signal.
 11. The innerfocus lens system according to claim 1, wherein the negative lens in thethird lens unit is a negative meniscus lens with a convex surface facingtoward the object side.
 12. The inner focus lens system according toclaim 1, wherein the negative lens and the positive lens in the firstlens unit are meniscus lens convex to the object side.
 13. The innerfocus lens system according to claim 1, wherein the followingconditional expressions (3) is satisfied:−0.3<f2/f1<0.3   (3) where f1 is a focal length of the first lens unit,and f2 is a focal length of the second lens unit.
 14. The inner focuslens system according to claim 1, wherein the following conditionalexpressions (4) is satisfied:0.02<Ds/f2<0.7   (4) where f2 is a focal length of the second lens unit,and Ds is a distance on an optical axis from an exit surface of thefirst lens unit to the aperture stop.
 15. The inner focus lens systemaccording to claim 7, wherein the following conditional expression issatisfied:−3<f3/f<−1.7.
 16. The inner focus lens system according to claim 8,wherein the following conditional expression is satisfied:0.9<f2/f<1.5.
 17. The inner focus lens system according to claim 13,wherein the following conditional expressions is satisfied:−0.2<f2/f1<0.
 18. The inner focus lens system according to claim 14,wherein the following conditional expressions is satisfied:0.1<Ds/f2<0.7.